An international research team has prepared a set of lanthanide antimony clusters that represent the first isolable compounds containing an all-metal antiaromatic ring. The achievement continues to expand the concept of aromaticity beyond its humble beginnings 150 years ago.
Researchers including Xue Min and Zhong-Ming Sun of Changchun Institute of Applied Chemistry and Ivan A. Popov and Alexander I. Boldyrev of Utah State University created a series of anions, [Ln(Sb ) ]3–, where Ln is La, Y, Ho, Er, or Lu. They made the anions by treating lanthanide benzyl complexes with the Zintl cluster complex K Sb in pyridine solvent and then isolating the anions as potassium cryptand salts.
On the basis of X-ray crystal structures and computational bonding analysis, the team says the rhombic Sb rings that serve as ligands to the lanthanide metals are antiaromatic (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201600706).
The concept of antiaromaticity has a storied history. In 1865, German chemist August Kekulé proposed the concept of aromaticity to explain the unusual properties of benzene, a planar carbon ring that exhibits high stability and low reactivity. In 1931, German chemist Erich Hückel added to the definition that aromatic compounds have a delocalized 4n﻿﻿﻿ + 2 π-electron system. In 1965, on the centennial of Kekulé’s proposal, Columbia University’s Ronald Breslow proposed the idea of antiaromaticity—the antonym of aromaticity—to characterize planar carbon rings with a 4n π-electron system that exhibit low stability and high reactivity.
Aromaticity and antiaromaticity were originally thought to be purely the domain of organic chemistry. But during the past 20 years, chemists have shown that this organic boundary is flexible. In 1995, Gregory H. Robinson and coworkers of the University of Georgia isolated the sodium salt of a phenyl-substituted Ga ring with two π﻿ electrons, introducing the concept of metalloaromaticity.
In 2003, Boldyrev’s group in collaboration with Lai-Sheng Wang, now at Brown University, followed suit by reporting Li Al –, which includes an antiaromatic Al 4– ring containing four π electrons. However, the gaseous molecule was created in a laser-based experiment and couldn’t be trapped in a condensed state.
With the [Ln(Sb ) ]3– series, chemists now have the first examples of isolable inorganic antiaromatic compounds. As a key feature, each Sb ring stabilized by the lanthanide metal has four delocalized π electrons. The Sb unit is analogous to cyclobutadiene, Boldyrev says, which is the quintessential antiaromatic organic compound.
“Antiaromaticity in these all-metal systems is very nice,” Breslow tells C&EN. “It is gratifying to see that our proposal, which was quite unexpected when we first made it for organic systems, has such generality.”
Further advances of aromaticity and antiaromaticity into metal territory will be valuable for understanding the properties of metal clusters, bulk metals, and alloys, Boldyrev and Sun add, which could be handy for making thin-film electronic materials.
“From a conceptual perspective, this is another example of the concept of aromaticity—in this case antiaromaticity or antimetalloaromaticity—being extended beyond the realm of carbon,” Robinson says. “More important, taking all of this work into consideration, aromaticity and metalloaromaticity seem to be foundational principles throughout the whole of chemistry.” This article has been translated into Spanish by Divulgame.org and can be found here.

Ink on paper can act as an electrode in a thin, flexible battery.
Inspired by Chinese brush painting, a team led by Xin-Bo Zhang at the Chinese Academy of Sciences' Changchun Institute of Applied Chemistry fabricated a flexible lithium-air battery using lithium foil and paper with a carbon-based ink (pictured). Electrons are stripped from the foil, creating lithium ions that flow to the inked paper electrode, where they combine with oxygen from the air. The resulting battery can hold a charge even after it has been bent 1,000 times. A foldable pack of four batteries, which weighs less than 2 grams, can supply current for 100 hours.
The technique paves the way for cheap and easily manufactured flexible batteries, the authors say.

Chinese scientists have developed a flexible lithium-based battery that is based on Chinese brush painting. More
Scientists in China have developed a flexible, rollable, foldable battery inspired by traditional Chinese calligraphy involving ink on paper.
Worldwide demand for flexible electronics is rapidly growing, because the technology could enable such things as video screens and solar panels to bend, roll and fold. These flexible electronics require batteries that are equally flexible to power them, but conventional batteries are too rigid and bulky to be used in flexible electronics.
Chinese scientists, however, have developed a flexible lithium-based battery that is based on Chinese brush painting. [5 Crazy Technologies That Are Revolutionizing Biotech]
Lithium-ion batteries power most portable devices, from smartphones to tablet computers to laptops. However, so-called lithium-air batteries could, in principle, hold five to 10 times as much energy as a lithium-ion battery of the same weight. This means that lithium-air batteries could theoretically give electric cars the same range as gasoline ones.
Batteries usually contain two electrodes — the anode and the cathode. In a lithium-air battery, the anode is generally made of lithium metal, while the cathode is typically a porous carbon material that allows the surrounding air into the battery. As the lithium reacts with oxygen in the air, it discharges electricity. Recharging the device reverses the process.
The scientists noted that the main component of black painting ink is carbon, and that paper is porous, thin, flexible, light and cheap. They reasoned that ink drawn on paper could serve as a cathode for a lithium-air battery in a very simple manner.
"Due to the ultra-high theoretical energy density of lithium-oxygen batteries, they may be one of the most suitable candidates in the future for the development of flexible electronics," study senior author Xinbo Zhang, a materials scientist at the Changchun Institute of Applied Chemistry in China, told Live Science.
The researchers constructed a battery from a sandwich of three layers — an ink-paper cathode, a sheet of lithium foil as the anode, and a sheet made of glass fibers between the anode and the cathode that permits electrically charged ions to flow between the cathode and anode.
Zhang and his colleagues found their prototype batteries possessed energy-storage capacities comparable to commercial lithium-ion batteries, even after 1,000 cycles of flexing back and forth. They could also easily fold these sheets into battery packs.
In the future, Zhang said he and his colleagues will explore lightweight flexible coatings for these batteries to protect them from corrosion.
Zhang and his colleagues detailed their findings in the Dec. 22 issue of the journal Advanced Materials.
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